Piezoelectric composite apparatus and a method for fabricating the same

ABSTRACT

A method for fabricating a piezoelectric macro-fiber composite actuator comprises making a piezoelectric fiber sheet by providing a plurality of wafers of piezoelectric material, bonding the wafers together with an adhesive material to form a stack of alternating layers of piezoelectric material and adhesive material, and cutting through the stack in a direction substantially parallel to the thickness of the stack and across the alternating layers of piezoelectric material and adhesive material to provide at least one piezoelectric fiber sheet having two sides comprising a plurality of piezoelectric fibers in juxtaposition to the adhesive material. The method further comprises bonding two electrically conductive films to the two sides of the piezoelectric fiber sheet. At least one conductive film has first and second conductive patterns formed thereon which are electrically isolated from one another and in electrical contact with the piezoelectric fiber sheet.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a divisional of pending U.S. patentapplication Ser. No. 09/430,677, filed Oct. 29, 1999.

ORIGIN OF THE INVENTION

[0002] The invention described herein was made by employees of theUnited States Government and may be used by or for the Government forgovernmental purposes without the payment of any royalties thereon ortherefor.

BACKGROUND OF THE INVENTION

[0003] 1. Field of the Invention

[0004] The present invention is generally related to piezoelectric fibercomposite strain actuators.

[0005] 2. Description of the Related Art

[0006] Conventional piezoelectric fiber composite actuators aretypically manufactured using a layer of extruded piezoelectric fibersencased in protective polymer matrix material. Interdigitated electrodesetched or deposited onto polymer film layers are placed on the top andbottom of the fibers to form a relatively thin actuator laminate.Protecting the fibers in a matrix polymer strengthens and protects thepiezoelectric material. The resulting package is more flexible andconformable than actuators formed from monolithic piezoelectric wafers.These actuators can be easily embedded within or placed upon non-planarstructures using conventional manufacturing techniques. In addition, theuse of interdigitated electrode poling permits production of relativelylarge, directional in-plane actuation strains. The directional nature ofthis actuation is particularly useful for inducing shear (twisting)deformations in structures.

[0007] Unfortunately, the methods of manufacturing conventionalpiezoelectric fiber composites typically use relatively high cost,extruded, round piezoelectric fibers. Moreover, alternative methods ofmanufacture using square fibers, which are milled from lower costmonolithic piezoelectric wafers, have been unsuccessful due to thedifficulty of aligning individual square fibers during actuator assemblywithout shifting and rolling. Rolled square fibers tend to expose sharpcorners and edges which can sever the interdigitated electrode layersduring the final process of actuator assembly. Both the round and squarefiber approaches require individual handling of piezoelectric fibersduring assembly, thereby resulting in relatively high manufacturingcosts.

[0008] Another disadvantage of conventional piezoelectric fibercomposite actuators is the requirement of relatively high operatingvoltages. High operating voltages are needed to produce electric fieldswhich are sufficiently strong to propagate through the protectivepolymer material encasing the piezoelectric fibers. These electrodevoltages are several times higher than those theoretically required toproduce a given strain in the unprotected piezoelectric material.Additionally, round fibers have a low contact area with the electrode,thereby causing losses and decreased efficiency. To compensate for theselosses, increased voltages are required. Conventional techniques forapplying electrodes directly in contact with the piezoelectric fibershave thus far not been practical.

[0009] It is therefore an object of the present invention to provide animproved piezoelectric fiber composite strain actuator and a method formaking same.

[0010] Still other objects and advantages of the present invention willin part be obvious and will in part be apparent from the specification.

SUMMARY OF THE INVENTION

[0011] The above and other objects and advantages, which will beapparent to one of skill in the art, are achieved in the presentinvention which is directed to, in one aspect, a method for fabricatinga piezoelectric macro-fiber composite actuator. The first step comprisesproviding a structure comprising piezo-electric material which has afirst side and a second side. First and second films are then adhesivelybonded to the first and second sides, respectively, of thepiezo-electric material. The first film has first and second conductivepatterns formed thereon which are electrically isolated from one anotherand in electrical contact with the piezo-electric material. In oneembodiment, the second film does not have any conductive patterns. Thefirst and second conductive patterns of the first film each have aplurality of electrodes that cooperate to form a pattern ofinterdigitated electrodes. In another embodiment, the second film has apair of conductive patterns similar to the conductive patterns of thefirst film.

[0012] In a related aspect, the present invention is directed to apiezoelectric macro-fiber composite actuator, comprising:

[0013] a structure consisting of piezo-electric material having a firstside and a second side;

[0014] a first film bonded to the first side of the structure, the filmfurther including first and second conductive patterns formed thereon,the first conductive pattern being electrically isolated from the secondconductive pattern, both conductive patterns being in electrical contactwith the piezo-electric material structure, the first and secondconductive patterns each having a plurality of electrodes that cooperateto form a pattern of interdigitated electrodes; and

[0015] a second film bonded to the second side of the structure.

[0016] In a further aspect, the present invention is directed to apiezoelectric macro-fiber composite actuator, comprising:

[0017] a plurality of piezoelectric fibers in juxtaposition, each fiberhaving a first side and a second side, each pair of adjacent fibersbeing separated by a channel;

[0018] a first adhesive layer disposed over the first sides of thefibers and in the channel;

[0019] a first film bonded to the first sides of the fibers, the filmfurther including first and second conductive patterns formed thereon,the first conductive pattern being electrically isolated from the secondconductive pattern, both conductive patterns being in electrical contactwith the piezo-electric material structure, the first and secondconductive patterns each having a plurality of electrodes that cooperateto form a pattern of interdigitated electrodes;

[0020] a second adhesive layer disposed over the second sides of thefibers and into the channels; and

[0021] a second film bonded to the second sides of the fibers, thesecond film having a first conductive pattern and a second conductivepattern electrically isolated from the first conductive pattern of thesecond film, the first and second conductive patterns of the second filmbeing in electrical contact with the fibers, the first and secondconductive patterns of the second film each having a plurality ofelectrodes that cooperate to form a pattern of interdigitatedelectrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The features of the invention are believed to be novel and theelements characteristic of the invention are set forth withparticularity in the appended claims. The figures are for illustrationpurposes only and are not drawn to scale. The invention itself, however,both as to organization and method of operation, may best be understoodby reference to the detailed description which follows taken inconjunction with the accompanying drawings in which:

[0023]FIG. 1 is a perspective view of a typical piezoelectric wafer.

[0024] FIGS. 2-7B are perspective views illustrating preferred methodsteps of the present invention for making a piezoelectric macro-fibercomposite actuator.

[0025]FIG. 8 is a top plan view of the assembled piezoelectricmacro-fiber composite actuator having electrically conductive extensionsattached thereto.

[0026]FIG. 9 is an exploded, perspective view illustrating an actuatorfabricated in accordance with an alternate embodiment of the method ofthe present invention.

[0027]FIG. 10 is an exploded, perspective view illustrating an actuatorfabricated in accordance with a further embodiment of the method of thepresent invention.

[0028]FIG. 11 is an exploded, perspective view illustrating an actuatorfabricated in accordance with yet another embodiment of the method ofthe present invention.

[0029]FIGS. 12A and 12B are perspective views illustrating an actuatorfabricated in accordance with yet a further embodiment of the method ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0030] In describing the preferred embodiments of the present invention,reference will be made herein to FIGS. 1-12B of the drawings in whichlike numerals refer to like features of the invention.

[0031] (1) Preferred Embodiment

[0032] Referring to FIG. 1, the first step of the method of the presentinvention entails providing a ferro-electric wafer 20. For example,wafer 20 is fabricated from unelectroded, piezoelectric material. In oneembodiment, PTZ-5 piezoelectric ceramic material is used to fabricatethe wafer 20. However, it is to be understood that any piezoelectricmaterial may be used to fabricate wafer 20. In a preferred embodiment,piezoelectric wafer 20 has a thickness between about 0.002 and 0.010inches.

[0033] Referring to FIG. 2, the next step entails disposingpiezoelectric wafer 20 on a relatively thin polymer backing sheet 22. Ina preferred embodiment, the polymer backing sheet is moderately adhesiveso as to facilitate handling during the subsequent steps of thefabrication method of the present invention.

[0034] Referring to FIG. 3, the next step comprises forming a pluralityof slots or channels 24 on piezoelectric wafer 20. While the slots 24extend through substantially the entire thickness of wafer 20, they donot completely slice the underlying polymer backing sheet 22. This stepresults in the creation of a sheet of side-by-side piezoelectricmacro-fibers 26 attached to the polymer backing layer 22. In a preferredembodiment, slots 24 are formed by a machining process that uses acommercially available computer-controlled dicing saw. However, othercutting methods may be used, e.g. lasers. In a preferred embodiment,each slot 24 has substantially the same width, which is between about0.001 and 0.005 inches. However, each slot 24 can have a width less than0.001 inch or greater than 0.005 inch. In a preferred embodiment, eachmacro-fiber 26 has a width between about one (1) and (2) two times thethickness of piezoelectric wafer 20. However, each macro-fiber 26 canhave a width that is less than the thickness of piezoelectric wafer 20or greater than twice the thickness of piezoelectric wafer 20.

[0035] Referring to FIG. 4, the next step is to fabricate electrically apair of non-conducting film elements that will be bonded to macro-fibers26. One such film element is film 28. Film 28 can be fabricated from anytype of electrically non-conducting material. In one embodiment, theelectrically non-conducting material is fabricated from a polyimide. Onesuitable material is Kapton® manufactured and marketed by Dupont®. In apreferred embodiment, film 28 has a thickness between about 0.0005 and0.001 inches. Preferably, film 28 has width and length dimensions whichare larger than the width and length of piezoelectric wafer 20. Thereasons for this configuration will be discussed below.

[0036] Referring to FIG. 4, film 28 comprises two electricallyconductive patterns 30 and 32. Conductive pattern 30 comprises alongitudinally extending portion 34 and interdigitated electrode fingers36. Conductive pattern 32 comprises a longitudinally extending portion38 and interdigitated electrode fingers 40. In one embodiment,conductive patterns or electrodes 30 and 32 are formed on film 28 usinga photo-resist-and-etch process and pre-bonded polyimide-copper sheetlaminate (e.g. Dupont® Pyralux® copper clad laminates). In a preferredembodiment, the thickness of the copper sheet material is between about0.0005 and 0.001 inches. For example, a copper sheet having a thicknessof about 0.0007 inch has provided good results. Although the foregoingdescription is in terms of conductive patterns 30 and 32 beingfabricated from copper-sheet material, other types of sheet materials,e.g. gold, silver, etc, may also be used. The polyimide-conductivematerial laminate may also utilize an electro-deposited conductive layerinstead of a pre-bonded conductive sheet, such as rolled and annealedcopper.

[0037] Referring to FIG. 4, in a preferred embodiment, thecenter-to-center spacing of longitudinally extending portions 34 and 38is about six times the thickness of piezoelectric wafer 20, and thespacing between interdigitated electrodes or “fingers” 36 and 40 isabout equal to the thickness of piezoelectric wafer 20. Thecenter-to-center spacing of longitudinally extending portions 34 and 38and interdigitated electrodes or fingers 36 and 40, however, can beother than described above. Furthermore, the width of conductivepatterns 30 and 32 may have any suitable width.

[0038] Referring to FIGS. 2-4, film 28 has width and length dimensionsthat are larger than the width and length of piezoelectric wafer 20 soas to permit the placement of longitudinally extending portions 34 and38 of conductive patterns 30 and 32, respectively, away frompiezoelectric wafer 20. This configuration significantly lessens thepotential for cracking of macro-fibers 26 caused by highly non-uniformelectrical field distribution in regions beneath and adjacent to thelongitudinally extending portions 34 and 38. Additionally, thispackaging concept affords a sealed electrical system protected from theenvironment.

[0039] Referring to FIGS. 4 and 5, a second film 42 is fabricated inaccordance with the steps described above. In one embodiment, film 42comprises conductive patterns or electrodes 44 and 46. Conductivepattern 44 comprises longitudinally extending portion 48 andinterdigitated electrodes or fingers 50. Similarly, conductive pattern46 comprises longitudinally extending portion 52 and interdigitatedelectrodes or fingers 54. Conductive patterns 44 and 46 of film 42 are“mirror images” of conductive patterns 30 and 32, respectively, of film28. The next step comprises positioning films 28 and 42 as shown in FIG.5 such that film 28 confronts one side or face of macro-fibers 26 andfilm 42 confronts the other side of macro-fibers 26. Conductive patterns30 and 32 of film 28 are directly aligned with conductive patterns 44and 46 of film 42. Thus, conductive patterns 30 and 32 are in“mirror-image” alignment with conductive patterns 44 and 46 across thethickness of macro-fibers 26. Although film 42 has been described in theforegoing description as having conductive patterns thereon, film 42 maybe configured without any conductive patterns.

[0040] Referring to FIGS. 6A, 6B, 7A, and 7B, films 28 and 42 are bondedwith an adhesive to macro-fibers 26 to form a flexible laminate. In apreferred embodiment, the adhesive is a two-part liquid epoxy to bondfilms 28 and 42 to macro-fibers 26. An example of such a liquid epoxy isScotchweld DP-460 epoxy manufactured by 3M Company. However, other typesof bonding materials can be used, e.g. urethane, acrylic, etc. Referringto FIG. 6A, the first step in the bonding process is to coat theelectrode face of film 42 with a relatively thin layer of liquid epoxy.Referring to FIG. 6B, sheet 22 and macro-fibers 26 are then placed onfilm 42 such that macro-fiber 26 contacts the epoxycoated face ofelectrode film 42. Light pressure, indicated by arrow 56, and heat areapplied in a vacuum to partially cure the epoxy layer to affix themacro-fibers to electrode film 42. After the partial cure is complete,polymer backing sheet 22, previously used for handling of macro-fibers26, is peeled away and discarded. Referring to FIG. 7A, macrofibers 26are now attached to the bottom electrode film 42 by the epoxy. Anadditional coat of liquid epoxy is now applied to macro-fibers 26 inorder to fill all machined slots 24 between adjacent fibers 26.Application of epoxy in this manner serves to substantially eliminateair pockets between adjacent, alternately charged electrode fingers 36,40, 50 and 54 in the final assembly. The elimination of these airpockets substantially reduces the probability of electrical arcing orpermanent shorts which would render the actuator inoperable.

[0041] Referring to FIGS. 6B, 7A, and 7B, after slots 24 are filled withthe epoxy, the next step is to apply a relatively thin coat of epoxy tothe electroded face of upper film 28. Next, film 28 is placed epoxy sidedown onto the previously coated surface of macro-fibers 26 such thatconductive electrode patterns 30, 32 and 44, 46 of films 28 and 42,respectively, are substantially aligned. The next step entails applyingmoderate pressure, indicated by arrow 58, and heat to the assembly offilms 28, 42 and macro-fibers 26. The heat and pressure are applied in avacuum until a substantially complete, void-free cure of the epoxy isattained. Application of this pressure also forces the relatively thickcopper conductive patterns or electrodes 30, 32 and 44, 46 to contactand rest upon the flat surfaces of the macro-fibers 26. Such contactbetween the relatively thick copper conductive patterns or electrodes30, 32 and 44, 46 and the flat surfaces of macro-fibers 26 creates abond line between the conductive patterns or electrodes 30, 32 and 44,46 and fiber 26 that is extremely thin or “starved,” resulting in only aminimal attenuation of the actuator's electric field produced whenvoltage is applied. The bond line between the unelectroded portions offilms 28 and 42 (i.e. the portions of films 28 and 42 having noconductive pattern) and fibers 26 is sufficiently thick to keep films 28and 42 attached. This process results in a longitudinal modepiezoelectric fiber actuator 10.

[0042] As shown in FIG. 8, conductive patterns 30 and 32 are providedwith electrically conductive extensions 68 and 70, respectively. Duringoperation, an external power supply (not shown) is electricallyconnected to the extensions 68 and 70 in a manner such that at any onemoment in time, opposite electrical polarity is supplied tointerdigitated fingers 36, 40 and 50, 54. This polarity generateselectric fields directed along the length of fibers 26 in the regionsbetween adjacent interdigitated electrode fingers 36 and 40 and betweenfingers 50 and 54.

[0043] The interdigitated electrodes 36, 40 and 50, 54 are also used forpolarizing the piezoelectric fibers 26. Polarization of the macro-fibers26 is typically required before operating the device as an actuator.Polarization is performed by applying a steady voltage across alternateelectrode fingers 36, 40 and 50, 54. In one embodiment, a voltage whichgenerates an average electric field intensity of approximately 300% ofthe room temperature coercive electric field of the macro-fibers 26 isused. Such voltage is applied to the actuator for approximately 20minutes at room temperature. Other poling techniques, as are wellunderstood in the art, may also be used.

[0044] Subsequent application of a voltage to conductive patterns 30,32, 44, and 46 produces an induced strain in macro-fibers 26. Thelargest strain produced occurs along the fiber length direction, with acontractile strain occurring in the transverse direction.

[0045] (2) Alternate Embodiments

[0046]FIG. 9 depicts an alternate piezoelectric fiber actuator 100 ofthe present invention. Shear-mode actuator 100 is configured to allowcontinuous twisting moments to be easily produced in a host structure,e.g. high aspect ratio structures, beams, spars, etc. Shear-modeactuator 100 generally comprises films 102, 104 and piezoelectric fibers106. Films 102, 104 and fibers 106 are adhesively bonded together usingan epoxy as described above. Piezoelectric fibers 106 have separatedslots 108 which are the result of a cutting or slicing process as hasbeen previously described. Fibers 106 define a longitudinally extendingedge 110. Slots 108 are formed at an angle with respect tolongitudinally extending edge 110. Preferably, each slot 108 is formedat a 45° angle with respect to the longitudinal extending edge 110because such an angular orientation provides optimum results in inducingpiezoelectric shear stresses within a host structure. However, slots 108may be formed at a different set of angles with respect to thelongitudinally extending edge 110.

[0047] Film 102 includes two conductive patterns 112 and 114 formedthereon. Conductive pattern 112 includes a longitudinally extendingportion 116 and interdigitated electrodes or fingers 118. Similarly,conductive pattern 114 includes a longitudinally extending portion 120and interdigitated electrodes or fingers 122. As shown in FIG. 9,fingers 118 are angulated with respect to longitudinally extendingportion 116. Similarly, fingers 122 are angulated with respect tolongitudinally extending portion 120. In a preferred embodiment, fingers118 and 122 are formed at a 45° angle with respect to portions 116 and120, respectively, so that fingers 118 and 120 are substantiallyperpendicular to the fibers 106.

[0048] In one embodiment, film 104 includes two conductive patterns 124and 126 formed thereon. Conductive pattern 124 includes a longitudinallyextending portion 128 and interdigitated electrodes or fingers 130.Similarly, conductive pattern 126 includes a longitudinally extendingportion 132 and interdigitated electrodes or fingers 134. As shown inFIG. 9, fingers 130 are angulated with respect to longitudinallyextending portion 128. Similarly, fingers 134 are angulated with respectto longitudinally extending portion 132. In a preferred embodiment,fingers 130 and 134 are formed at a 45° angle with respect to portions128 and 132, respectively, so that fingers 130 and 134 are substantiallyperpendicular to the fibers 106. Although film 104 has been described inthe foregoing description as having conductive patterns thereon, film104 may also be configured without any conductive patterns. Films 102and 104 are bonded with an adhesive to macro-fibers 106 in a processsimilar to the process previously described for assembly ofpiezoelectric fiber actuator 10 and shown by FIGS. 6A, 6B, 7A, and 7B.

[0049] Actuator 100 further includes four electrical conductors (notshown) wherein each electrical conductor is electrically connected to acorresponding one of conductive patterns 112, 114, 124, and 126. In apreferred embodiment, each of the electrical conductors are positionednear the edge of films 102, 104 and function to electrically connectactuator 100 to external electronic circuitry (not shown). The fourelectrical conductors apply electrical power to actuator 100 in the samemanner as described above.

[0050]FIG. 10 illustrates a further embodiment of the actuator of thepresent invention. Actuator 200 generally comprises a plurality ofpiezoelectric macro-fibers 202 separated by slots 204, and films 206,208, 210, and 212. Slots 204 are formed by the slicing or cuttingmethods previously described herein. Films 206 and 208 are generally thesame in construction as films 28 and 42, respectively, discussed above.

[0051] Film 206 includes two conductive patterns 214 and 216 formedthereon. Conductive pattern 214 includes a longitudinally extendingportion 218 and interdigitated electrodes or fingers 220. Similarly,conductive pattern 216 includes a longitudinally extending portion 222and interdigitated electrodes or fingers 224. As shown in FIG. 10,fingers 220 and 224 are substantially perpendicular to longitudinallyextending portions 218 and 222, respectively.

[0052] In one embodiment, film 208 comprises two conductive patterns 226and 228. Conductive pattern 226 includes a longitudinally extendingportion 230 and interdigitated electrodes or fingers (not shown).Similarly, conductive pattern 228 includes a longitudinally extendingportion 232 and interdigitated electrodes or fingers 236. The fingers offilm 208 are substantially perpendicular to longitudinally extendingportions 230 and 232. Film 208 may also be configured without anyconductive patterns.

[0053] Actuator 200 further comprises anisotropically conductive filmsor sheets 210 and 212 positioned on the top and bottom of piezoelectricmacro-fibers 202. Each film 210 and 212 has generally the same surfacearea as the total surface area of piezoelectric macro-fibers 202. Films210 and 212 are used to bond films 206 and 208 to the piezoelectricmacro-fibers 202. Each film 210 and 212 comprises athermoset/thermoplastic adhesive matrix. In one embodiment, the adhesivematrix has a thickness between about 0.0001 and 0.002 inches. Theadhesive matrix has randomly loaded conductive particles. Theseconductive particles provide conductive paths through the thickness ofthe adhesive film, but not through the plane of the film. This pathingarrangement permits the fingers of films 206 and 208 to be in directelectrical contact with the underlying piezoelectric fibers 202 whileremaining electrically isolated from adjacent, oppositely chargedfingers. In one embodiment, the conductive particles have a diameter ofabout 0.0005 inch. Films 210 and 212 comprise Z-Axis Film, product no.3M 5303R, manufactured by 3M Company, Inc. However, other films havinggenerally the same anisotropically conductive characteristics as theaforementioned Z-Axis Film may be used.

[0054] Referring to FIG. 10, before final assembly of actuator 200,slots 204 are filled with an electrically non-conductive matrix epoxy toprevent the development of air pockets. The application of the epoxy isimplemented in generally the same manner as previously described forassembly of actuator 10.

[0055] Referring to FIG. 10, the use of films 210, 212 to bond films 206and 208 to piezoelectric macro-fibers 202 creates relatively strong bondlines that are maintained beneath and between fingers of films 206 and208. In an alternate embodiment, films 206 and 208 may be added duringthe fabrication of the shear-mode actuator previously described andshown in FIG. 9.

[0056]FIG. 11 shows another embodiment of the actuator of the presentinvention. Actuator 300 generally comprises a monolithic piezoelectricwafer 302 and films 304 and 306. Wafer 302 may be produced as alongitudinal-mode or shear-mode actuator. Films 304 and 306 haveelectrode patterns and are generally the same in construction as films28 and 42 described above and shown in FIGS. 4 and 5.

[0057] Film 304 comprises a conductive pattern 308 which has alongitudinally extending portion 310 and interdigitated electrodes orfingers 312. Film 304 further comprises conductive pattern 314, whichhas a longitudinally extending portion 316 and interdigitated electrodesor fingers 318. As shown in FIG. 11, fingers 312 and 318 aresubstantially perpendicular to longitudinally extending portions 310 and316, respectively.

[0058] In one embodiment, film 306 comprises a conductive pattern 320having a longitudinally extending portion 322 and interdigitatedelectrodes or fingers 324. Film 306 further comprises a conductivepattern 326 having a longitudinally extending portion 328 andinterdigitated electrodes or fingers 330. As shown in FIG. 11, fingers324 and 330 are substantially perpendicular to the longitudinallyextending portions 322 and 328, respectively. Film 306 may also beconfigured without any conductive patterns.

[0059] Films 304 and 306 may be bonded to wafer 302 by any of themethods previously described. The omission of the machined slots inwafer 302 significantly reduces the per-unit cost of actuator 300 andprovides a relatively high actuation-efficiency device. Additionally,the lamination effect of the attached electrode films 304 and 306provides actuator 300 with a predetermined degree of flexibility andconformability which, although not as great as actuators 10, 100 and200, makes actuator 300 suitable for applications wherein endurance andfatigue life are not major considerations, for example, launch vehiclepayload shrouds, torpedo bodies, missile stabilizer fins, etc.

[0060] A further embodiment of the actuator of the present invention isgiven in FIGS. 12A and 12B. The first step in fabricating actuator 400is to bond together a plurality of relatively thin piezoelectric wafers402 to form a stack 404. In a preferred embodiment, a liquid epoxy aspreviously described is used to bond together the wafers 402. Stack 404may be of almost any height. In one embodiment, the height of stack 404is about 0.25 inch. In a preferred embodiment, the thickness of bondlines 406 between adjacent wafers 402 is between about 0.125 and 0.25times the nominal thickness of the individual piezoelectric wafers 402.After stack 404 is bonded, it is cured at relatively moderate pressureand temperature to form a substantially void-free bonded stack. In apreferred embodiment, the aforementioned pressure and temperature areapplied under a vacuum.

[0061] Next, stack 404 is sliced parallel to the thickness direction andalong the length direction, as indicated by dotted lines 408, to providea plurality of relatively thin, piezoelectric sheets 410. In oneembodiment, a wafer dicing saw is used to cut fiber sheets 410. However,other cutting methods may be used. Fiber sheets 410 may be handled andpackaged in the same manner as monolithic piezoelectric wafers. In oneembodiment, the thickness of each sheet 410 is about equal to thethickness of one of the piezoelectric wafers 402 used to form stack 404.However, each sheet 410 may have a thickness that is less than orgreater than the thickness of one of the piezoelectric wafers 402.

[0062] Referring to FIG. 12B, sheet 410 is positioned between films 412and 414. Film 412 comprises a conductive pattern 416, which has alongitudinally extending portion 418 and interdigitated electrodes orfingers 420, and a conductive pattern 422, which has a longitudinallyextending portion 424 and interdigitated electrodes or fingers 426. Asshown in FIG. 12B, fingers 420 and 426 are substantially perpendicularto longitudinally extending portions 418 and 424, respectively.

[0063] Film 414 comprises a conductive pattern 428 having alongitudinally extending portion 430 and interdigitated electrodes orfingers 432. Film 414 further comprises a conductive pattern 434 havinga longitudinally extending portion 436 and interdigitated electrodes orfingers 438. Fingers 432 and 438 are substantially perpendicular tolongitudinally extending portions 430 and 436, respectively. Film 414may also be configured without any conductive patterns. Films 412 and414 are adhesively bonded to sheet 410 via a liquid epoxy or using ananisotropically conductive film as previously described.

[0064] The configuration shown in FIGS. 12A and 12B has two significantadvantages. First, the possibility of bonding to a surface skin isvirtually eliminated. Second, all the macro-fibers of sheets 410 arepre-aligned.

[0065] (3) Advantages Over Prior Art Actuators And Methods

[0066] The method of the present invention substantially eliminates theneed to manufacture and individually handle large numbers ofpiezoelectric fibers. Thus, production time and handling costsassociated with packaging piezoelectric fiber composite actuators aresignificantly reduced. The method of the present invention is easilycontrolled and precise, which greatly enhances the repeatability anduniformity of the actuators produced. The method of the presentinvention permits square fibers to be manufactured and easily alignedwithin the actuator package without the possibility of damage to theactuator electrodes. Thus, the difficulties associated with the use ofsquare cross-section piezoelectric fibers are virtually eliminated. Theuse of square fibers in accordance with the present invention instead ofround fibers allows the volume fraction of piezoelectric material withinthe actuator package to be increased, thereby improving the actuationstress capability of the actuator. The use of the relatively thickcopper conductive patterns, which are attached via liquid epoxy oranisotropically conductive adhesive, also provide for an unimpededelectrical connection to be made between the piezoelectric material andthe electrodes. As a result, the electric field transfer efficiency ofthe actuator electrodes is significantly improved, which in turnincreases the strain produced per unit applied voltage. A furtheradvantage is that the square or rectangular fibers have a substantiallyflat contact area with the electrodes. This flat contact area isrelatively greater than the contact area achieved with round fibers.

[0067] The polyimide films each have width and length dimensions thatare larger than the width and length of piezoelectric wafer so as topermit the placement of longitudinally extending portions of theconductive patterns (e.g. portions 34 and 38 of conductive patterns 30and 32, respectively) away from the piezoelectric wafer. Thisconfiguration significantly lessens the potential for cracking of themacro-fibers caused by highly nonuniform electrical field distributionin regions beneath and adjacent to the longitudinally extending portionsof the conductive patterns. Additionally, this packaging concept affordsa sealed electrical system that is protected from the environment.

[0068] While the present invention has been particularly described, inconjunction with a specific preferred embodiment, it is evident thatmany alternatives, modifications, and variations will be apparent tothose skilled in the art in light of the foregoing description. It istherefore contemplated that the appended claims will embrace any suchalternatives, modifications, and variations as falling within the truescope and spirit of the present invention.

What is claimed is:
 1. A method of fabricating a piezoelectric compositeapparatus, comprising the steps of: providing a plurality of wafers ofpiezoelectric material; bonding the wafers together with an adhesivematerial to form a stack of alternating layers of piezoelectric materialand adhesive material, the stack having a thickness; cutting through thestack in a direction substantially parallel to the thickness of thestack and across the alternating layers of piezoelectric material andadhesive material to provide at least one piezoelectric fiber sheetcomprising a plurality of piezoelectric fibers in juxtaposition toadhesive material, the at least one piezoelectric fiber sheet having afirst side and a second side; providing a first film having a firstconductive pattern and a second conductive pattern formed thereon, thefirst conductive pattern being electrically isolated from the secondconductive pattern, the first and second conductive patterns each havinga plurality of electrodes that cooperate to form a pattern ofinterdigitated electrodes; providing a second film; bonding the secondfilm to the second side of the at least one piezoelectric fiber sheet;and bonding the first film to the first side of the at least onepiezoelectric fiber sheet such that the conductive patterns of the firstfilm electrically contact the piezoelectric fibers of the at least onepiezoelectric fiber sheet.
 2. The method according to claim 1 whereinthe wafer of piezoelectric material comprises a monolithic piezoelectricmaterial.
 3. The method according to claim 1 wherein each piezoelectricfiber has a substantially rectangular cross-section.
 4. The methodaccording to claim 1 wherein at least one of the conductive patterns ismade of copper.
 5. The method according to claim 1 wherein the secondfilm has a first conductive pattern and a second conductive pattern, thefirst conductive pattern of the second film being electrically isolatedfrom the second conductive pattern of the second film, the first andsecond conductive patterns of the second film each having a plurality ofelectrodes that cooperate to form a pattern of interdigitatedelectrodes, and the step of bonding the first film further comprises thestep of positioning the first film so that the conductive patterns ofthe first film are substantially aligned with the conductive patterns ofthe second film.
 6. The method according to claim 5 further comprisingattaching electrically conductive extensions to the first and secondconductive patterns of the first film and attaching electricallyconductive extensions to the first and second conductive patterns of thesecond film.
 7. The method according to claim 1 wherein the step ofbonding the second film further comprises applying an epoxy to the atleast one piezoelectric fiber sheet.
 8. The method according to claim 5wherein the first film and the second film each have a longitudinallyextending axis and the step of cutting produces at least onepiezoelectric sheet having a plurality of piezoelectric fibers thatextend in the direction of the longitudinal axes of the first and secondfilms.
 9. The method according to claim 8 wherein each interdigitatedelectrode of the first and second conductive patterns extends in adirection that is substantially perpendicular to the longitudinallyextending axes of the first and second films and substantiallyperpendicular to the longitudinally extending direction of the pluralityof piezoelectric fibers.
 10. The method according to claim 1 wherein thesecond film has a first side and a second side, and the step of bondingthe second film comprises: applying an adhesive layer to the first sideof the second film; placing the first side of the second film on thesecond side of the at least one piezoelectric fiber sheet; and curingthe adhesive layer.
 11. The method according to claim 10 wherein thefirst film has a first side and a second side, and the step of bondingthe first film comprises: applying a second adhesive layer to the firstside of the first film; placing the first side of the first film on thefirst side of the at least one piezoelectric fiber sheet; and curing thesecond adhesive layer.
 12. The method according to claim 1 furthercomprising the step of attaching electrically conductive extensions tothe first and second conductive patterns.
 13. A method of fabricating aplurality of piezoelectric fibers, comprising the steps of: providing aplurality of wafers of piezoelectric material; bonding the waferstogether with an adhesive material between each wafer to form a stack ofalternating layers of piezoelectric material and adhesive material, thestack having a thickness; and cutting through the stack in a directionsubstantially parallel to the thickness of the stack and across thealternating layers of piezoelectric material and adhesive material toprovide at least one piezoelectric fiber sheet comprising a plurality ofpiezoelectric fibers in juxtaposition to the adhesive material.
 14. Themethod according to claim 13 wherein the wafer of piezoelectric materialcomprises a monolithic piezoelectric material.
 15. The method accordingto claim 13 wherein each piezoelectric fiber has a substantiallyrectangular cross-section.
 16. A piezoelectric composite apparatus madeby a process comprising: providing a plurality of wafers ofpiezoelectric material; bonding the wafers together with an adhesivematerial to, form a stack of alternating layers of piezoelectricmaterial and adhesive material, the stack having a thickness; cuttingthrough the stack in a direction substantially parallel to the thicknessof the stack and across the alternating layers of piezoelectric materialand adhesive material to provide at least one piezoelectric fiber sheetcomprising a plurality of piezoelectric fibers in juxtaposition toadhesive material, the at least one piezoelectric fiber sheet having afirst side and a second side; providing a first film having a firstconductive pattern and a second conductive pattern formed thereon, thefirst conductive pattern being electrically isolated from the secondconductive pattern, the first and second conductive patterns each havinga plurality of electrodes that cooperate to form a pattern ofinterdigitated electrodes; providing a second film; bonding the secondfilm to the second side of the at least one piezoelectric fiber sheet;and bonding the first film to the first side of the at least onepiezoelectric fiber sheet such that the conductive patterns of the firstfilm electrically contact the piezoelectric fibers of the at least onepiezoelectric fiber sheet.
 17. The apparatus according to claim 16wherein the wafer of piezoelectric material comprises a monolithicpiezoelectric material.
 18. The apparatus according to claim 16 whereineach piezoelectric fiber has a substantially rectangular cross-section.19. The apparatus according to claim 16 wherein at least one of theconductive patterns is made of copper.
 20. The apparatus according toclaim 16 wherein the second film has a first conductive pattern and asecond conductive pattern, the first conductive pattern of the secondfilm being electrically isolated from the second conductive pattern ofthe second film, the first and second conductive patterns of the secondfilm each having a plurality of electrodes that cooperate to form apattern of interdigitated electrodes, and the step of bonding the firstfilm further comprises the step of positioning the first film so thatthe conductive patterns of the first film are substantially aligned withthe conductive patterns of the second film.
 21. The apparatus accordingto claim 20 further comprising attaching electrically conductiveextensions to the first and second conductive patterns of the first filmand attaching electrically conductive extensions to the first and secondconductive patterns of the second film.
 22. The apparatus according toclaim 16 wherein the step of bonding the second film further comprisesapplying an epoxy to the at least one piezoelectric fiber sheet.
 23. Theapparatus according to claim 20 wherein the first film and the secondfilm each have a longitudinally extending axis and the step of cuttingproduces at least one piezoelectric fiber sheet having a plurality ofpiezoelectric fibers that extend in the direction of the longitudinalaxes of the first and second films.
 24. The apparatus according to claim23 wherein each interdigitated electrode of the first and secondconductive patterns extends in a direction that is substantiallyperpendicular to the longitudinally extending axes of the first andsecond films and substantially perpendicular to the longitudinallyextending direction of the plurality of piezoelectric fibers.
 25. Theapparatus according to claim 16 wherein the second film has a first sideand a second side, and the step of bonding the second film comprises:applying an adhesive layer to the first side of the second film; placingthe first side of the second film on the second side of the at least onepiezoelectric fiber sheet; and curing the adhesive layer.
 26. Theapparatus according to claim 25 wherein the first film has a first sideand a second side, and the step of bonding the first film comprises:applying a second adhesive layer to the first side of the first film;placing the first side of the first film on the first side of the atleast one piezoelectric fiber sheet; and curing the second adhesivelayer.
 27. The apparatus according to claim 16 wherein the process formaking the piezoelectric composite apparatus further comprises the stepof attaching electrically conductive extensions to the first and secondconductive patterns.
 28. A plurality of piezoelectric fibers made by aprocess comprising: providing a plurality of wafers of piezoelectricmaterial; bonding the wafers together with an adhesive material betweeneach wafer to form a stack of alternating layers of piezoelectricmaterial and adhesive material, the stack having a thickness; andcutting through the stack in a direction substantially parallel to thethickness of the stack and across the alternating layers ofpiezoelectric material and adhesive material to provide at least onepiezoelectric fiber sheet.
 29. The plurality of piezoelectric fibersaccording to claim 28 wherein the wafer of piezoelectric materialcomprises a monolithic piezoelectric material.
 30. The plurality ofpiezoelectric fibers according to claim 28 wherein each piezoelectricfiber has a substantially rectangular cross-section.